Enhanced stability of hierarchical zeolite-metal bifunctional catalysts for conversion of levulinic acid to valeric biofuels

[Display omitted] •A hierarchical HZSM-5 with interconnected meso-macropores was employed to support Ni catalysts.•The hierarchical porosity alters the acid site distribution and reduces the strong acid sites.•The catalytic lifetime is prolonged from 20 h to 180 h for conversion of levulinic acid to ethyl valerate.•The hierarchical Ni/HZSM-400 catalyst can be regenerated 4 times with a total lifetime of 900 h.•Addition of 10% ethyl valerate in diesel can effectively decrease the exhaust pollutants of engine. Valeric ester has been recognized as a new generation of biofuels with promising environmental advantages. Metal-zeolite bifunctional catalysts demonstrated outstanding performance in tandem transformation including the tandem conversion of levulinic acid to valeric ester via synergetic catalysis of C = C bond hydrogenation and C-O bond cleavage, but still suffering from rapid catalyst deactivation due to coke formation. Herein, a hierarchical HZSM-5 supported Ni catalyst was developed, and displayed efficient ethyl valerate production activity with a catalytic lifetime as long as 180 h, which is 8-fold longer than that of microporous zeolite counterpart (20 h). Notably, this nonprecious Ni catalyst can be stably regenerated by calcination at least four times with its total lifetime up to 900 h. Systematic investigations reveal that the presence of secondary porosity, the less strong acid sites and weaker adsorption of ethyl valerate on hierarchical catalysts could effectively alleviate coke formation and promote catalyst stability. More importantly, the hierarchical porosity of HZSM-5 alters the acid site distribution, enabling more acid sites to be located on the external surface, and thus retards the coverage of the active site by carbonaceous species. Besides, the addition of 10 % ethyl valerate in diesel can reduce the emission of exhaust pollutants, wherein the CO, NOx, HC, and soot emissions can be reduced by 13 %, 5 %, 4 %, and 27 % at an engine speed of 1500 rpm, respectively. This work develops an alternative strategy by tuning pore size and acid distribution to enhance the metal/zeolites catalyst stability against coke deposits and promotes the practical application of ethyl valerate as an environmentally friendly oxygenated additive for the current liquid fuels.